1. Field of the Invention
The present invention relates to a position measuring device for measuring a position using a change in the inductance of a coil, which is suitable as a microflowmeter which is capable of measuring a flow rate in, for example, the range from 10 to 100 [ml/min.], or a length measuring unit for measuring a length of a piston rod as it is displaced into or out of a sleeve, a float-type flowmeter, and a position measuring method.
2. Description of the Related Art
There has been a demand in the market for a float-type flowmeter which is capable of electrically, instead of visually, detecting the position of a float in a fluid. This float-type flowmeter is needed in applications where the float position cannot be visually recognized when the fluid is deeply colored, or when the float position is detected for automatically controlling the rate of the fluid.
One conventional float-type flowmeter for electrically detecting the position of a float in a fluid is disclosed, for example, in Japanese laid-open patent publication No. 10-38644.
The disclosed float-type flowmeter has a float whose position is variable depending on the rate of a fluid that flows through a barrel tube. The barrel tube is made of a metal and the float is made of a magnetically permeable material. A primary coil and a secondary coil wound around the barrel tube and the float which serves as a magnetic core, make up a proportional variable differential transformer for electrically detecting directly the position of the float.
The disclosed float-type flowmeter, however, is unable to measure a desired flow rate unless the float has a length greater than the full length of the coils. Also, the proportional variable differential transformer is complex in structure.
A rod having a linear succession of minute magnets, a so-called Magnescale, may be used as a length measuring unit for measuring a length of a piston rod as it is displaced into or out of a sleeve. However, using this kind of length measuring unit is expensive and the circuit arrangement becomes complex.